A number of developmental and adult brain disorders are associated with midbrain dopamine neurons (mDNs), including Parkinson's disease (PD), schizophrenia, autism, dyskinesias, and drug addiction. Thus, the fundamental mechanisms that regulate the development and function of these cells are of great import. In prior studies, we and others have investigated basic regulatory processes, such as regulation of the expression of key enzymes in the dopamine biosynthetic pathway in the context of dopamine neuron development, function, and survival. However, it is clear from these studies that there is a high level of complexity governing all of these processes. For instance, at least 2 transcription factors, Nurr1 and Pitx3, function synergistically to govern expression of late developmental mDN markers in simplified ES cell-based culture models. More recently, in preliminary data and a published manuscript, my lab has found evidence of an added layer of complexity involving post-transcriptional regulation by microRNAs (miRNAs) in the context of mDN development and function. miRNAs are evolutionarily conserved, 18-25 nucleotide non-protein coding transcripts that play an important function in post-transcriptional regulation of gene expression during development. Specifically, we identified a microRNA, miR-133b, that is enriched in mDNs and functions within a regulatory feedback circuit with Pitx3. Here we propose to more broadly define the level of complexity of gene expression regulation by miRNA in mDNs, and to determine the function of these forms of regulation in vivo. Ultimately, such forms of regulation are likely to play a role in mDN-associated diseases, and furthermore manipulations of these mechanisms offer potential avenues for therapies. We wish to test two hypotheses: 1. miRNAs function in the regulation of mDNs, both within feedback circuits with mDN transcription factors and by the direct regulation of key mDN targets. 2. Such regulatory networks play functionally important roles in mDNs in vivo.